This invention relates to a method for forming a closed end on a tube of heat-shrinkble film, and also to a closed end tube formed by the method.
In conventional methods for forming a tube of heat-shrinkable film, such as a thermoplastic resin film with a closed end, a piece of heat-shrinkable film tube is placed over a cylindrical mold which has a generally planar top end, with part of the tube extending above the top end. The tube is then heated, so as to shrink it, so that the part extending above the top end of the mold contracts considerably and conforms roughly thereto. Then this part is pressed against the top end of the mold by a heated plate. Thereby the end of the closed tube is flattened and positively sealed. This can be used for forming a container, a cap, a cap seal, or the like.
However, this prior art method has several defects described as follows.
(1) Since, prior to the pressing by the heated plate, the top portion of the tube is folded up on the flat mold top, especially towards the center thereof, high pressure must be exerted by the heated plate in order to form a tube end of uniform thickness, and in practice this tube end tends not to be uniform, especially towards its center.
(2) Excess melted film squeezed out from between the heated plate and the top surface of the mold tends to drip down the sides of the tube during the pressure stage of the process, and this results in a finished article of poor appearance. This is particularly noticeable because the pressure applied to the heated plate has to be high, as explained with reference to reason (1).
(3) Since, as explained with reference to reason (1), the closed end of the finished product tends to be of non-uniform thickness, and since it is formed while being heated, the finished article may be distorted during the cooling of the film.
(4) For reasons (1) and (3), it is difficult to print on the outer surface of the closed end.
(5) After cooling, the parts of the closed end where the melted resin has flowed substantially in the radial direction sometimes crack.